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$$
 E = 2 \pi \epsilon \left[ \frac{2}{5} \left(\frac{\sigma}{r}\right)^{10} - 
                       \left(\frac{\sigma}{r}\right)^4 -
                       \frac{\sqrt(2)\sigma^3}{3\left(r+\left(0.61/\sqrt(2)\right)\sigma\right)^3}\right]
                       \qquad r < r_c
$$

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